Resin compositions such as a methacrylic ester polymer are obtained from a polymer composition produced by a continuous polymerization according to which a raw material monomer, a polymerization initiator, etc., are continuously supplied to a reactor to be polymerized. A continuous solution polymerization method using a solvent (or a dispersion medium, that will also apply hereinafter) to conduct a continuous polymerization, and a continuous bulk polymerization method using no solvent to conduct a continuous polymerization are known as the continuous polymerization methods.
A continuous polymerization apparatus is known that uses two reactors connected in series to each other, to conduct a continuous polymerization using the production method of a polymer composition. Patent Document 1 proposes a production method according to which the most part of the polymerization is conducted in the pre-stage reactor, and the polymerization is completed and the polymerization initiator, and the like, are removed in the post-stage reactor, and describes that the post-stage reactor is supplied with an additional comonomer, and, when necessary, an additional chain transfer agent through a delay addition line. Patent Document 2 proposes another production method according to which the polymerization is conducted to some extent in the pre-stage reactor, and a solvent is added to the content in the post-stage reactor to conduct a polymerization, and describes that, when necessary, the post-stage reactor is supplied with an additional copolymerizable vinyl compound and chain transfer agent through an additional raw material line. Patent Document 3 proposes a production method according to which the reactors are configured in two stages and the average residence time for each of these reactors is set to be within a predetermined range for a half-life period of the polymerization initiator, and describes that the monomer mixture in the raw material composition may be supplied to the pre-stage reactor while a part thereof may be side-fed to the post-stage reactor. Patent Documents 1 to 3 all relate to a continuous solution polymerization method.